The manufacture of large scale integrated circuit chips requires extremely complex equipment and stringent procedures in order to maintain production losses at an acceptable level. Such losses are frequently due to defects in one or more of the various masks used in the manufacturing process. These defects are usually in the nature of a clear spot in an area which should be opaque or an opaque spot in an area which should be clear. As circuit complexity increases and device geometries become smaller, the effect of photomask quality upon final yield becomes increasingly significant.
Visual inspection of these photomasks is quite time consuming and yields mixed results due to inherent subjectivity and varying operator skill levels. As integrated circuits become larger and more complex, such visual inspection becomes extremely difficult, if not impossible. The need for automated inspection systems was recognized resulting in the development of several such systems. Typical of these automated inspection systems is the Mask Analysis system disclosed in U.S. Pat. No. 4,218,142 which issued Aug. 19, 1980 to Kryger et al. Kryger et al. utilizes a laser driven scanning spot which scans a particular chip area of the mask comparing the signal received from the scanning device with a data base containing standard data representing an ideal mask.
A more modern and widely used automated inspection system is the KLA-101 series automatic photomask inspection system which is manufactured by KLA Instruments Corp. of Santa Clara, California. The KLA-101 machine, which utilizes an image scanning system, simultaneously scans two different die areas on the same mask. All die areas on a given mask should contain identical patterns. If a difference is detected between two die areas, comparison with a third will determine which die area is defective. Alignment of the machine with the photomask to be inspected is accomplished manually by visually aligning each of the two sets of optics on their respective die areas. Identical features of the two die areas are selected so that the optics are imaging nominally identical patterns. As the photomask is scanned by the system, the scanning signal is gated to exclude signals generated while the scanner is outside of the die area. The die area is commonly referred to within the industry as the care area.
Occasionally, the KLA machine will become misaligned and scan slightly outside of the care area thereby giving erroneous indications of defects and failing to detect actual defects occurring within the care area. Heretofore, there has been no reliable and simple way for an operator to determine whether or not the machine is scanning outside of the desired care area. The present invention permits an operator to easily make this determination by means of a novel test mask.